In the first four years of this project, we studied the role of oxytocin in mediating affiliative behaviors, and using autoradiographic techniques, we investigated the physiologic regulation of oxytocin receptors. These various studies have demonstrated the importance of gonadal steroids for inducing oxytocin receptor binding. Indeed, findings from behavioral and receptor binding studies have suggested that the induction of peptide receptors may be an important mechanism by which estrogen, progesterone, or androgens regulate behavior. During this past year, we focused further on the details of the gonadal steroid-oxytocin receptor interaction by investigating the physiologic and behavioral consequences of oxytocin receptor induction. In one set of receptor binding studies, we determined that testosterone increases oxytocin receptor binding in the ventromedial nucleus (VMN) of the hypothalamus through the combined actions of both of its metabolites, dihydrotestosterone and estrogen. In a collaborative study, electrophysiologic responses to oxytocin increased in hypothalamic slices from animals treated with estrogen. Injection of oxytocin directly into the VMN was found to increase affiliative behavior in female rats receiving threshold doses of estrogen and progesterone. The physiologic importance of oxytocin for the neural mediation of female reproductive behavior was assessed by central injection of an oxytocin antagonist with various gonadal steroid treatment regimens. The oxytocin antagonist decreased sexual receptivity in females primed with both estrogen and progesterone. Data collected over the past four years are consistent in suggesting a role for oxytocin in the mediation of several forms of attachment or affiliative behavior. As a new approach to the central question of this project, the neurobiology of attachment, during this past.year we began a systematic comparative analysis of oxytocin receptor distribution in species selected for differences in social attachment strategies. Monogamous, biparental rodents such as the prairie vole were found to have markedly different patterns of oxytocin receptors compared to polygamous congeners such as the montane vole, We are currently investigating the relationship of these species differences in peptide receptors to differences in behavior.